1. Field of the Invention
The present invention relates to a new class of polymerizable photoinitiators, especially suitable for food compliant radiation curable formulations, and methods for preparing the photoinitiators.
2. Description of the Related Art
A free radical photoinitiator initiates the polymerization of monomers when exposed to actinic radiation by the formation of a free radical. Photoinitiators are frequently used in UV-curable compositions, such as UV-curable inkjet inks.
Two types of free radical photoinitiators can be distinguished. A Norrish Type I initiator is an initiator which cleaves after excitation, yielding the initiating radical immediately. A Norrish Type II-initiator is a photoinitiator which is activated by actinic radiation and forms free radicals by hydrogen abstraction from a second compound that becomes the actual initiating free radical. This second compound is called a co-initiator or polymerization synergist.
A photoinitiator can be a monofunctional compound, but can also be a multifunctional compound, i.e. having more than one photoinitiating group. WO 03/033492 (COATES BROTHERS) discloses multifunctional thioxanthone photoinitiators.
When radiation curable compositions are used for food packaging, toys and dental applications, the amount of extractable residues is a critical issue and needs to be minimized. Low molecular weight compounds are usually not completely built into the polymer network and are prone to be readily extracted or to diffuse out of the cured composition. Therefore, it is a continuous concern to design photoinitiators having a reduced tendency to be extracted or to migrate out of the cured composition.
One approach to minimize the extraction of photoinitiators is the use of photoinitiators with a higher molecular weight. However, polymeric initiators have a tendency to lose reactivity. Hence, often considerable amounts of polymeric initiators are required in order to reach the desired curing speed, thereby also increasing the viscosity to an undesirable level for a great number of applications using radiation curable compositions, such as e.g. inkjet printing. To overcome the undesirable viscosity increase of radiation curable compositions, EP 1616921 A (AGFA GRAPHICS) and EP 1674499 A (AGFA GRAPHICS) disclose radiation curable compositions comprising polymeric photoinitiators, comprising a dendritic polymer core. While the use of a dendritic polymer core is advantageous for maintaining a low viscosity of the radiation curable composition, a further improvement in curing speed is desirable, especially in the absence of nitrogen inertisation.
Another approach in solving the extraction problem is the design of a photoinitiator having one or more ethylenically unsaturated polymerizable groups so that it can be copolymerized with the other monomers of the radiation curable composition. Numerous photoinitiators comprising an ethylenically unsaturated polymerizable group have been disclosed in the literature, for use in radiation curable compositions or as a monomer for the preparation of polymeric photoinitiators.
JP 2004-224993 (NIPPON KAYAKY) discloses self-photopolymerization type photopolymerization initiators for reducing its evaporation or sublimation from cured films of radiation curable compositions. Other (meth)acrylated thioxanthones have been disclosed in, for example, CA 2005283 (BASF) and CA 1180486 (CIBA).
(Meth)acrylated benzophenones are disclosed in, for example, US 2006/0142408 (NATIONAL STARCH) and GB 925117 (DU PONT).
(Meth)acrylated a-hydroxy-ketones are disclosed in, for example, WO 2005/108452 (ASHLAND), WO 97/17378 (COATES BROTHERS) and EP 538553 A (HUGHES AIRCRAFT).
(Meth)acrylated a-amino ketones are disclosed in, for example, WO 96/20919 (CIBA) and CA 2005283 (BASF).
(Meth)acrylated acyl phosphine oxide initiators are disclosed in, for example, WO 2006/056541 (CIBA), WO 2004/103580 (CIBA) and AU 2003205731 (BASF).
(Meth)acrylated benzil dialkyl acetals are disclosed in JP 2005-082679 (DAINIPPON INK).
Often, the synthesis of the target photoinitiators requires the use of (meth)acryloyl chloride. It is commonly known that (meth)acryloyl chloride is highly reactive and limited in stability. It is often contaminated with cyclic dimers (see for example in JP 2002-187868 (DAICEL CHEMICAL)) thus requiring distillation prior to use. Combined with the highly toxic nature of (meth)acryloyl chloride, the limited availability on the market and the high cost, synthetic methods using (meth)acryloyl chloride are not well suited for the preparation of (meth)acrylated photoinitiators on an industrial scale.
PCT/EP2008/066289 (AGFA GRAPHICS) discloses an optimized synthetic method for the preparation of acrylated or methacrylated photoinitiators. Though avoiding highly toxic and unstable reagents, an isolation procedure is still required to obtain high purity (meth)acrylated photoinitiators, suitable for food packaging applications, generating additional costs in production and increasing the ecological footprint.
Therefore, there is still a need for photoinitiators, having a significantly reduced tendency to be extracted or migrate out of the cured composition, having a good compatibility with a wide variety of radiation curable formulations and accessible via a simple and cost efficient synthetic procedure having a reduced ecological footprint.